THERMAL TREATMENTS WITH A CURLING IRON fi g ure 6. Conrinued. 2 7 with a curling iron on the fatigue behavior of untreated and conditioner-treated hair fibers. Untreated hair fibers and fibers treated with 0.5% aqueous solutions of Polyquaternium 10 (PQl0) at 40 ° C and cetyl trimethylammoniumbromide (CETAB) at 50°C for ten minutes, followed by five one-minute rinses in deionized water, were allowed to condition at 21 °C and 65 % RH. The hair was held under normal tension (no load applied) for five minutes over a preheated Philips curling iron (-l l0 ° C) without overlapping the hair, before being gently unwound and allowed to recondition. Single

24 JOURNAL OF COSMETIC SCIENCE Table I Values for the Mechanical Properties of Thermally Treated Hair Initial Extension to Load at Work to Total Post-yield modulus break break 20% work modulus (GN/m2) (%) (GN/m2) Olm2) 01m2) (GN/m2) Untreated hair 3.09 50.00 0.191 607163 1979180 0.301 Curled hair 3.07 47.34 0.203 621560 1931227 0.386 hair fibers were mounted for the fatigue experiment in TRI's hair fatigue tester, in which the cycling speed was set at the rate of 2.3 Hz with a load of 40 g. The fatigue failure data, treated according to the Weibull statistics (1), yield a hazard parameter known as characteristic life. Mathematically it represents the number of cycles necessary to break -63% of the test specimens. Damage results in a reduction of characteristic life. Figure 7 compares the characteristic life (0) of dry untreated and conditioner-treated hair fibers before and after curling for five minutes with a curling iron. Surprisingly, an effect of thermal treatment could be established. Hair fibers clearly showed a higher charac­ teristic life (0) after thermal treatment with the curling iron, indicating improved fatigue resistance. It is possible that this is due to some crosslinking in the interior of the hair fiber as a result of dehydration at high temperature. The presence of the conditioning compounds appears to enhance this thermally induced crosslinking in the form of salt linkages and hydrophobic bonding, leading to a significant increase in fatigue resistance (characteristic life {0]). The improvements in fatigue properties of an ionomeric polymer by the introduction of ,-. 30000 e JI Before Curling ■ After Curling � 25000 :J fJ 20000 ·c: 15000 fJ 10000 5000 0 Untreated 1xPQ-10 10xPQ-10 1xCETAB 10xCETAB Hair Treatment Figure 7. Comparing characteristic life (0) of dry untreated and conditioner-treated hair fibers before and after prolonged curling for five minutes under normal tension with a curling iron.